Apple TCP/IP Dangers

Kenn's
Tech
Notes

19980404: Apple TCP/IP Dangers

April/1998
Tech Note #4

For any technicians with Apple network printers and/or Macintoshes on their LANs, there is an aspect of MacTCP which you should be aware of. Whereas in Windows, we have the choice of configuring TCP statically (by assigning a specific address to each machine by hand), or dynamically (allowing a DHCP server to assign addresses automatically), Apple has a third option which is a potential problem for other systems on the network.

In MacTCP, there are three options for assigning the IP address. Again, you can assign it a static address (which we normally don't do). The other options are "server" and "automatic". Never use the "automatic" option. The "automatic" option was an Apple-proprietary extension to the TCP/IP standard and was designed for use on networks which were exclusively Macintosh. If you make the mistake of selecting "automatic", what will happen is the workstation will poll the network when it first powers up to see what addresses are in use. It will then select an address that is not in use at that moment. The problem arises when such a system chooses an address at a time when another system - which either has a static address or a DHCP-assigned address - is not powered up. If the Apple device picks the address belonging (rightfully) to the powered-down workstation, the workstation that owns the address won't have TCP/IP functionality when it is booted up later!

Apple's Tech Info Library Article 22001 explains this in more detail, and confirms that the Open Transport TCP/IP, which ships with Macintosh System 7.5.3 and newer Mac OSs, does not use this method.

However, we have seen instances of Apple printers using this method - and causing problems - when installed as a network node (as opposed to being shared from a node). If you install an Apple printer as a network node, assign it an IP address statically and make sure that address is either outside the range of your DHCP scope, or exclude the address from the scope manually, using DHCP Manager.

DISCLAIMER: This document is intended for the reference of computer support personnel within Winnipeg School Division No. 1. There is no warranty or liability if procedures recommended here have an adverse affect on any systems. Use them at your own risk. Any trademarks mentioned are the property of their owners, none of whom have certified any information provided here. Opinions expressed here are personal only and do not represent the policy of Winnipeg School Division No. 1 or any other organization anywhere.

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2001 June 9

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